1. Field of the Invention
A tandem line is provided to convert raw materials into a high frequency coaxial cable unit having a substantially stress-free corrugated laminated outer conductor in which the corrugations are symmetrical and uniform in size and do not contribute to undesirable variations in impedance of the coaxial cable unit. The coaxial cable unit possesses the desired physical and electrical characteristics with reduced potential for high voltage breakdowns between inner and outer conductors and relatively low structural return losses.
2. Description of the Prior Art
In high frequency transmission systems, such as those which employ coaxial cable units as the conductors, it is extremely important to maintain matched impedance between all components. The impedances of a coaxial unit must be precisely matched to the impedance of a repeater or other terminating devices connected to the coaxial unit. The impedance of a coaxial unit is related to the ratio between the diameters of the inner and outer conductors.
Inner conductors of coaxial units are, for the most part, drawn copper wire, the diameter of which can be controlled very closely. The outer conductors, on the other hand, are metallic tubes which, in most cases, are formed from flat metallic strips. In order to provide an efficient transmission system, it becomes extremely important to control precisely the diameter of the tubular outer conductor and to eliminate as much as possible any stress within the tubular structure which might cause distortion of the tubular conductor after it is formed.
In manufacturing coaxial cable units with corrugated outer conductors formed of steel, copper and polymer laminates, the laminate was formed by heating the three components while continuously maintaining them under pressure in a rolling operation. Since the two metallic components, steel and copper, had different coefficients of thermal expansion, the laminated material would tend to curl after being cooled to ambient temperature. Forming a tubular outer conductor of a laminate having such a curl would result in undesirable internal stresses being left in the outer conductor.
Another situation resulting in improper stress concentration within the outer conductor developed when the laminated material was corrugated in a roller-type corrugator. Corrugators used in the manufacturing systems are those which employ driven corrugating rollers to pull the laminated material into position between the rollers as the rollers are turned.
Corrugated material emerging from such a corrugator very often exhibits a distorted shape in the tooth profile. The tooth profile takes on a "saw tooth" appearance instead of a symmetrical shape and this asymmetry causes undesirable, nonuniform stresses within the outer conductor.
Still another cause of undesirable stressing within the outer conductor develops when the corrugated, laminated material is formed into exact sized tubular shape which is required in the outer conductor and the seam is secured by soldering or welding. In forming any metallic structure into a curved shape, such as a cross section of a tube, each stage of formation has associated with it a certain amount of inherent spring back of the metal. When a tube is secured at its seam, as is the case in the subject type of coaxial cable unit, and the inherent spring back is not allowed to relax, the spring back force is maintained as inner stress within the closed tubular outer conductor.
Another factor which will adversely affect the constancy of diameter of the outer conductor is a stretching out of corrugations after the corrugations have been impressed in the laminated outer conductor material. Such a stretching of corrugations will occur if there is a failure to coordinate the velocities of a capstan which is pulling a completed coaxial unit and a system which is driving rollers of a forming mill that is being used to form the outer conductor into its tubular shape.
If the forming mill lags the capstan in speed, the difference in linear velocities of the device will be taken up by a stretching of the coaxial units. If the coaxial unit is stretched, the overall height of the corrugations is reduced and, thus, there is a decrease in the diameter of the coaxial unit and a consequential change in the characteristic impedance of the unit.
In putting together a coaxial unit of the type described, it is extremely important to keep the inner components of the coaxial unit free of contaminants such as oil or dust because of the increased probability of high voltage breakdowns which would occur because of the presence of such contaminants. In order to reduce the level of contamination within the coaxial unit, it is current practice to clean the center conductor thoroughly before introducing it into the coaxial unit.
Previous practice in the field of manufacturing coaxial cable units was one in which wires for inner conductors were subject to meticulous cleaning so that all wire drawing lubricant residue was removed from the surface of the wire and this cleaning was done in an operation separate from actual fabrication of the coaxial cable unit. As a result of using separate operations, the wire for center conductors was wound on reels and was passed over various guide pulleys before being insulated with discs and finally assembled into a coaxial cable unit. As a result of meticulous cleaning, which left the wire surface free of any lubrication and subsequent abrasive action between the wire surface and adjacent sections of the wire on reels and the various guide pulleys over which the wire was passed, there developed some chafing of the wire which, at times, caused sharp splinters and/or flakes of copper to exist on the surface of the wire. The sharp splinters and/or flakes of copper were places where an increased probability for high voltage breakdown between the inner conductor and the outer conductor of the coaxial cable unit existed. Also, these splinters and/or flakes are a means for generating corona when a voltage less than breakdown is applied to the coaxial cable unit. The presence of corona at or near the polyethylene insulating discs will create oxalic acid which will eventually oxidize the polyethylene disc and create a high voltage breakdown.
Another problem which develops in the manufacture of coaxial cable units is that of consistently applying insulating discs to the center conductor at high speeds. The present state of the art with respect to application of discs to inner conductors is one in which discs are punched in an operation separate from an in-line fabrication of the coaxial unit. The punched discs are batch-loaded into a hopper and are conveyed by vibratory and gravitational feeding systems to an insertion device. The existence of static charge on the surface of the discs very often results in the discs not feeding properly. This causes spaces to develop along the inner conductor where no discs are placed. These spaces manifest themselves as undesirable impedance discontinuities.
The prior art discloses methods of making metalclad, plywood panels having a desired configuration at ambient temperature. A hot bonded panel will tend to warp or bow after cooling below the bonding temperature because of the different forces exerted on opposite sides of the core by metal members having different thicknesses and metallurgical properties. The metal members may be selected so that the relationship of the forces exerted by the metal members on the core are controlled in a predetermined manner. In the alternative, forces are applied to the fully or partially cooled panel to permanently deform one of the metal members and limit the stress developed by that member. This may be accomplished by applying the force to the panel supported along spaced edges or by passing the panel through a roller arrangement wherein one roller is arranged in seating relationship with a spaced pair of rollers.